1. Field of the Invention
The present invention relates to a process for molding plastic articles and more particularly relates to a process for molding plastic covers for containers and to the plastic covers so produced.
2. Description of the Prior Art
In my copending patent application, Ser. No. 310,322 filed Dec. 4, 1972, which issued Aug. 21, 1973 as U.S. Pat. No. 3,753,511, there is disclosed a container and enclosure or cover therefor which has a number of distinct advantages. For example, with this particular container and cover the use of separate sealing means such as sealing foils for obtaining an airtight seal is not required. Also, with a relatively small total height of the cover member, and considerable production tolerances, a vigorous clamping and pressure effect can be realized so that a sealing action may be obtained which suffices for all practical requirements. This is particularly important to withstand any rough handling, such as may occur during transportation or manipulation of the containers, for example, in self-service stores.
The container and enclosure of cover therefor disclosed in U.S. Pat. No. 3,753,511 comprises shoulder close to the upper end of the wall and a push-in cover member having a circumferential raised rim portion provided with a circumferential skirt member depending externally over the container wall. The skirt member has its inner wall surface provided with a projection which engages underneath the shoulder on the container wall when the cover member is pushed over the open top of the container. A circumferential notch on the outer side of the skirt member provides a weakening line making it possible to tear off a lower portion of the skirt member including the projection when the container is to be opened but leaving the remaining cover member intact to be reusable for closing the container. The cover has a bulged portion on the outer surface of the inner wall, and there is a shaped contoured fit between the container and the cover.
While the aforementioned cover and container have great advantages for packaging various materials, the method of manufacturing the cover in particular has been relatively speaking, complicated and expensive. This has been due particularly to the fact that it was not possible to use a simplified die tool design in which the various elements could be moved in an axial direction only. More particularly, in the previous design, at least one die which was split radially had to be opened in a radial direction. This resulted in a number of disadvantages. The surfaces which sealed, moved against each other causing wear and a decrease of sealing ability with the result that the dies subject to radial opening movement had to be replaced more frequently than dies performing axial movement only. Also, due to the split in the radial direction, additional seams appeared on the molded product. Furthermore, in addition to the control elements employed with the axial-moving dies, special and different control elements had to be employed for controlling the radially opening dies.
There are, of course, many different techniques known for molding various products, including plastic materials. For example, U.S. Pat. Nos. 3,193,884 3,319,703, 3,344,942, 3,555,606, 3,584,356 and 3,687,591 are exemplary of such molding apparatus. However, the problem that the art faced prior to the present invention was how to design a container and cover therefor that had the aforementioned advantages while at the same time being able to manufacture the product, and particularly the cover, in a simple and inexpensive manner.